The present invention relates to a remote maintenance apparatus that is intended to increase the power of combined laser beams which are used in the disassembly of a vacuum wall of a fusion device.
In general, a plasma fusion device wherein a high-temperature plasma is confined within a vacuum wall and a nuclear fusion reaction is generated therein has a configuration such as that shown abbreviated in FIG. 9. A fusion device 1 has a vacuum wall 3 formed of a plurality of segments that create a vacuum boundary around a plasma 2. A toroidal coil 5 and a poloidal coil 6 are arranged around the outside of the vacuum wall 3, and the plasma 2 is confined within the vacuum wall 3 by the electromagnetic force of the toroidal coil 5 and the poloidal coil 6. The fusion device 1 is also provided with an impurity removal device 7 for expelling impurities generated within the plasma 2 to the outside, with a vacuum exhaust duct 8 connected thereto.
Of these structural members, the impurity removal device 7 is subjected to a high-temperature load from the plasma 2 and it is highly likely to receive damage due to impacts from neutrons. Therefore an operator of an impurity removal device 7 that is already installed must consider periodic disassembly and reassembly during the service lifetime of the fusion device 1.
Since the impurity removal device 7 is likely to be damaged by the thermal load received from the plasma, the arrangement is such that it is supplied with cooling water from a cooling piping system 9. Therefore, in order to periodically disassemble the impurity removal device 7, the cooling piping system 9 must be isolated and cut away beforehand, and a connection method such as welding must be used after the reassembly of the impurity removal device 7, to connect the cooling piping system 9. There are various problems involved with this procedure, such that it is not possible to ensure sufficient maintenance space in the peripheral areas of the plasma fusion device for this and similar work of cutting and reconnecting. In particular, with a fusion device intended to generate large levels of energy, since the size of the plasma is set to be relatively large in comparison with the entire body of the device, as shown in FIG. 9, the cooling piping system 9 has to be stacked within a limited space, and thus it is difficult to ensure enough maintenance space around the piping.
A proposed solution to this problem is to use a laser beam from within the cooling piping to cut the piping to disassemble it, or reassemble the piping by laser welding. In other words, a laser processing device 10 is installed in a movable manner within the cooling piping system 9, as shown in FIG. 10, and a laser beam 11 emitted from the outside is guided onto a work site 14 via a focusing lens 12 and a full mirror 13, to enable operation at the work site 14.
Considerations of the amount of water to be used dictate that the diameter of the above-described cooling piping is between 40 mm and 200 mm, the thickness thereof is governed by the water pressure and is thus 3 mm to 15 mm, and the power output required of the laser beam 11 is approximately 3 kW to 25 kW. Therefore, in order to use lasers to cut or weld this cooling piping system 9, a number of laser beam generators 15, each capable of generating a laser beam within the range of approximately 3 kW to 25 kW, must be positioned around the periphery of the fusion device 1 to correspond to the impurity removal device 7, as shown in FIG. 11.
The toroidal coil 5 and vacuum wall 3 shown in FIG. 9 are basic structural members making up the reactor core of the fusion device 1 and, in a conventional fusion device 1, these basic structural elements are not designed to be subject to maintenance or replacement.
However, if by some chance one of these basic structural members should become defective, and if these members are not designed to be maintainable or replaceable, the entire fusion device 1 will be disabled. Accordingly, if the original design did not take periodic replacement into consideration, it is necessary to consider the disassembly and reassembly of these basic structural elements. In particular, it is almost inevitable that the vacuum wall 3 itself could become damaged or defective, and toroidal coil 5 could be damaged too, so it will be necessary to disassemble the vacuum wall into separate segments in order to remove and replace the toroidal coil 5. That is why there is interest in developing a fusion device of a design that takes into consideration the assembly and disassembly of the vacuum wall thereof.
FIG. 12 is a partial plan view of a typical example of replacing a toroidal coil 5 by disassembling part of a vacuum wall 3 into individual segments 3a, 3b, . . . , then pulling the toroidal coil 5 and the segment 3a out in the direction of the arrow. The fusion device shown in FIG. 12 makes use of a design that leaves plenty of room in the space within a toroidal plasma, to facilitate disassembly and assembly. However, recent increases in the energy generated by fusion devices have dictated that they are designed so that the size of the plasma is relatively large in comparison with that of the entire device, as shown in FIG. 9.
FIG. 13 is an external view of a case of such a high-power type of fusion device using a segment structure for the vacuum wall. This fusion device 1 is also configured of a large number of segments 3a, 3b, 3c, . . . , and it is provided with a toroidal coil 5, a poloidal coil 6, and vacuum exhaust ducts 8. In this case, since there is some space around the outside of the torus, the segments can be connected together in a mechanical fashion by a means such as nuts and bolts. Within the torus, since the segments 3a, 3b, 3c, . . . , are in mutual contact, there is virtually no space for installing the nuts and bolts, nor for the work of tightening and removing them. That is why methods are being developed for this type of high-power fusion device to enable disassembly or reassembly by cutting or welding the vacuum wall 3 from the inside, in order to disassemble or reassemble the vacuum wall.
In other words, as shown in FIG. 14, in order to connect the segments 3a, 3b, 3c, . . . from the inside, they are welded together at weld portions 3m. Any suitable method can be used for welding the weld portions 3m, such as ordinary arc welding such as TIG or MIG welding, electron beam welding using a high-energy beam, or laser welding. However, arc welding using an arc plasma of a low energy density, such as TIG or MIG welding, has disadvantages such as it not only takes a great deal of time for the welding process, the materials are highly likely to become distorted by the heat, and, particularly when a single segment is reassembled, the fusion device cannot be re-created in the same condition as that at its initial assembly, and the functionality of the entire system can thus be lost. It is difficult to adapt electron beam welding to operations in atmosphere so it is usually done in a vacuum chamber. Thus, with a structure of dimensions of ten to several tens of meters, it is usual to adopt a partial vacuum electron beam welding method that makes use of a partial vacuum device for maintaining an evacuated region only at the area to be welded. However, with this partial vacuum electron beam welding method, it is difficult to maintain vacuum seal portion thereof, which makes this method unsuitable for reassembly welding in a fusion device where the welding is expected to be highly reliable and trustworthy.
On the other hand, the laser welding method is a high-energy beam welding method having the same high welding capability as the electron beam welding method, but welding distortion can be restrained, and this method can also be used in the reassembly welding of a fusion device.
The configuration of a system that uses the laser welding method to weld between segments from within the vacuum wall is shown in FIG. 15. A laser beam transmission system 17 formed of a beam duct connected to a laser beam generator 16 is guided into the interior of a vacuum wall 3 through a maintenance port 18, a laser beam work head 19 linked to the tip of the laser beam transmission system 17 is manipulated to reach a work site, a laser beam 20a generated from the laser beam generator 16 is transmitted via full mirrors 21a and 21b up to the laser beam work head 19, and the laser beam work head 19 is moved in a suitable manner to perform a prescribed job.
The laser beam work head 19 is provided with a focusing optical system 22 for focusing the laser beam 20a within a main work head body 19a, and the laser beam focused by the focusing optical system 22 is emitted from a operating nozzle at the tip, to irradiate a prescribed work site A, as shown in FIG. 16. To supplement the laser work, the main work head body 19a is also provided with an assist gas introduction opening 24 for introducing an assist gas to protect the optical system from spattering metal and metal vapor during the work.
A laser beam transport system shown in FIG. 17 comprises robotics 25 that support the laser beam transmission system 17 formed of the full mirrors and beam duct, and a robot guide used up until the robotics 25 get into the vicinity of the work site. The laser beam is focused at a prescribed energy density by the laser beam work head 19 via the laser beam transmission system 17, to perform an operation such as welding on the prescribed work site A.
The thus-configured laser welding system for welding between segments from within the vacuum wall is used to weld the surface of the vacuum wall, but structural requirements dictate that the thickness of this wall is between 8 mm and 40 mm, or even more, so a laser beam with an output power of approximately 5 kW to 50 kW is necessary. If the system is designed in such a manner that oxygen, nitrogen, or a mixture thereof is used in addition to the assist gas comprising Ar, He, or a mixture thereof that is used during welding, this system can also be used without modification as a laser cutting system.
As described above, the laser processing system makes remote maintenance both feasible and efficient, when it is used in either the disassembly/reassembly of cooling piping for an impurity removal device of a fusion device, or in the disassembly/reassembly between segments of a vacuum wall.
However, in contrast to the wall thickness of the cooling piping which is between 3 mm and 15 mm, the thickness of the vacuum wall is between 8 mm and 40 mm, so the ranges of laser outputs necessary for working with these two thicknesses differ as 3 kW to 25 kW and 5 kW to 50 kW. Therefore, laser beam generators of differing power outputs must be installed around the periphery of the fusion device, as shown in FIG. 18. In other words, laser beam generators 30a and 30b of an output power of 5 kW to 50 kW for cutting or welding between segments from within the vacuum wall must be provided near corresponding maintenance ports 18a and 18b of the fusion device 1, and also laser beam generators 31a, 31b, . . . of an output power of 3 kW to 25 kW for cutting or welding while the cooling piping of the impurity removal device is being disassembled or reassembled must be provided at suitable positions around the periphery of the fusion device 1.
Since this impurity removal device is designed on the assumption that it will undergo periodic disassembly and reassembly during the operating lifetime of the fusion device, the cooling piping is also expected to undergo periodic disassembly and reassembly, so the laser beam generators 31a, 31b, . . . for cutting and welding the piping are used periodically. In other words, a prespecified usage frequency can be expected of these laser beam generators.
However, the system for cutting or welding between segments from within the vacuum wall is provided in case the vacuum wall itself or the basic structural elements such as the toroidal coil should by some mischance become damaged or defective, so the likelihood that the laser beam generators 30a and 30b are used is extremely low. In addition, since these laser beam generators 30a and 30b must put out approximately twice the power output than the other laser beam generators 31a, 31b, . . . for the cooling piping, the laser beam generators 31a, 31b, . . . , cannot be used to replace the laser beam generators 30a and 30b for the vacuum wall. Therefore, expensive dedicated laser beam generators must be provided for the vacuum wall, despite them having such an extremely low operating efficiency.
Thus, in order to prepare for any kind of problem that may occur in a basic structural element, a fusion device remote maintenance system that provides dedicated high-power laser beam generators in the vicinity of maintenance ports in order to disassemble and reassemble segments from within the vacuum wall must be provided. This is a loss from the cost point of view and also the space in the vicinity .of the maintenance ports required for placing the high-power laser beam generators is lost. In addition, the provision of laser beam generators that will hardly ever be used could cause further problems such as release of laser gases and the inconvenience of output windows, so that there are problems such as the lasers must be activated at suitable intervals, even when they are not needed.